Near‐surface velocity structure from borehole and refraction seismic surveys

1994 ◽  
Author(s):  
Darryl Parry ◽  
Don C. Lawton
Keyword(s):  
Velocity Structure ◽  
Surface Velocity ◽  
Near Surface ◽  
Seismic Surveys ◽  
Refraction Seismic

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

2012 ◽  
Vol 227-228 ◽  
pp. 50-60 ◽  
Author(s):  
Laurence Perrier ◽  
Jean-Philippe Métaxian ◽  
Jean Battaglia ◽  
Esline Garaebiti
Keyword(s):  
Velocity Structure ◽  
Surface Velocity ◽  
Volcanic Complex ◽  
Near Surface

Turning‐ray tomography

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1917-1929 ◽  
Author(s):  
Joseph P. Stefani
Keyword(s):  
Velocity Structure ◽  
Initial Point ◽  
Real Data ◽  
Point Sources ◽  
Surface Velocity ◽  
Inversion Algorithm ◽  
Low Velocity ◽  
Linear Inversion ◽  
Near Surface ◽  
Velocity Inversion

Turning‐ray tomography is useful for estimating near‐surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning‐ray tomography in its capacity to estimate absolute near‐surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities. The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite‐ difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard‐rock overthrust structure with a low‐velocity zone and pinchouts, and a contemporaneous near‐shore marine trench filled with low‐ velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one‐ fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/datuming applications can also use these velocities. The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers. Turning‐ray tomography estimates near‐surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.

scholarly journals Crustal velocity structure of the Superior and Grenville provinces of the southeastern Canadian Shield

1997 ◽  
Vol 34 (8) ◽  
pp. 1167-1184 ◽  
Author(s):  
S. Winardhi ◽  
R. F. Mereu
Keyword(s):  
Greenstone Belt ◽  
Velocity Structure ◽  
Canadian Shield ◽  
Crustal Thickening ◽  
Surface Velocity ◽  
Data Set ◽  
Tectonic Zone ◽  
Near Surface ◽  
Abitibi Greenstone Belt ◽  
Sudbury Structure

The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.

Seismic Microzonation using 6C Measurements

2020 ◽  
Author(s):  
Sabrina Keil ◽  
Joachim Wassermann ◽  
Heiner Igel
Keyword(s):  
Velocity Structure ◽  
Building Design ◽  
Wave Dispersion ◽  
Velocity Profiles ◽  
Surface Velocity ◽  
Intelligent Building ◽  
Near Surface ◽  
Velocity Models ◽  
Single Station ◽  
Risk Areas

<p>Microzonation is one of the essential tools in seismology to mitigate earthquake damage by estimating the near surface velocity structure and developing land usage plans and intelligent building design. The number of microzonation studies increased in the last few years as induced seismicity becomes more relevant, even in low risk areas. While of vital importance, especially in densely populated cities, most of the traditional techniques suffer from different short comings. The microzonation technique presented here tries to reduce the existing ambiguity of the inversion results by the combination of single-station six-component (6C) measurements, including three translational and three rotational motions, and more traditional H/V techniques. By applying this new technique to a microzonation study in Munichs (Germany) inner city using an iXblue blueSeis-3A rotational motion sensor together with a Nanometrics Trillium Compact seismometer we were able to estimate Love and Rayleigh wave dispersion curves. These curves together with H/V spectral ratios are then inverted to obtain shear wave velocity profiles of the upper 100 m. The resulting 1D velocity profiles are used to estimate the local shaking characteristics in Munich. In addition, the comparison between the estimated velocity models and the borehole-derived lithology gives a positive correlation, indicating the applicability of our method.</p>

scholarly journals LIQUEFACTION RISK ASSESSMENT BY THE USE OF GEOPHYSICAL TECHNIQUES: THE TEST AREA OF NAFPLION CITY, GREECE

2017 ◽  
Vol 43 (3) ◽  
pp. 1438
Author(s):  
V.K. Karastathis ◽  
P. Karmis ◽  
T. Novikova ◽  
Z. Roumelioti ◽  
E. Gerolymatou ◽  
...  
Keyword(s):  
Risk Assessment ◽  
Cost Effective ◽  
Shallow Aquifer ◽  
Surface Velocity ◽  
Design Decision ◽  
Near Surface ◽  
Investigation Area ◽  
Seismic Methods ◽  
Refraction Seismic ◽  
Geophysical Techniques

An efficient and cost effective site characterization, with regard to the seismic hazard and liquefaction risk assessment, was accomplished with the aid of geophysics in the area, where the Nafplion city of Greece is expanding. The methodology adopted includes the recognition of the possible earthquake sources of the wider region, their modelling, in order to stochastically simulate the strong ground motion at the investigation area, and finally the calculation of the liquefaction risk. The investigation area was suspected of high liquefaction potential since the foundation ground consists of loose sandy silt with very shallow aquifer. The geophysical techniques considerably contributed to the detection and characterization of possible local seismic faults with the implementation of gravity and seismic methods. Special emphasis was given to the seismic depth migration and particularly to the construction of valid velocity models, in order to precisely calculate the dips of the possible faults. Additionally the geophysical techniques provided the near surface velocity structure for the calculation of the amplification of the seismic motion up to the surface, also required for the final estimation of the liquefaction risk. The seismic methods (seismic reflection, seismic refraction, seismic modelling, MASW, multichannel analysis of microtremors and crosshole investigations), if combined with geo-technical borehole testing, enhance their reliability and cover large areas in a cost-effective way in comparison with the standard borehole tests. In Nafplion area, evidence was found for a low factor of safety against liquefaction at specific sites within the study area. The results show that liquefaction probability can reach 80% at some sites depending on selected earthquake scenario, mainly at depths between 5 and 10 meters. This should be considered as highly important information for making risk-based design decision in this region.

Joint refraction traveltime tomography and migration for multilayer near-surface imaging

Geophysics ◽  
2019 ◽  
Vol 84 (6) ◽  
pp. U31-U43
Author(s):  
Yihao Wang ◽  
Jie Zhang
Keyword(s):  
High Velocity ◽  
Velocity Structure ◽  
Joint Inversion ◽  
Velocity Model ◽  
Surface Velocity ◽  
Inversion Method ◽  
Near Surface ◽  
Traveltime Tomography ◽  
First Arrival ◽  
Migration Method

In near-surface velocity structure estimation, first-arrival traveltime tomography tends to produce a smooth velocity model. If the shallow structures include a weathering layer over high-velocity bedrock, first-arrival traveltime tomography may fail to recover the sharp interface. However, with the same traveltime data, refraction traveltime migration proves to be an effective tool for accurately mapping the refractor. The approach downward continues the refraction traveltime curves and produces an image (position) of the refractor for a given overburden velocity model. We first assess the validity of the refraction traveltime migration method and analyze its uncertainties with a simple model. We then develop a multilayer refraction traveltime migration method and apply the migration image to constrain traveltime tomographic inversion by imposing discontinuities at the refraction interfaces in model regularization. In each subsequent iteration, the shape of the migrated refractors and the velocity model are simultaneously updated. The synthetic tests indicate that the joint inversion method performs better than the conventional first-arrival traveltime tomography method with Tikhonov regularization and the delay-time method in reconstructing near-surface models with high-velocity contrasts. In application to field data, this method produces a more accurately resolved velocity model, which improves the quality of common midpoint stacking by making long-wavelength static corrections.

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